This invention relates to high-voltage capacitors and the material employed for fabricating such capacitors. Techniques for acrylate deposition and adhesion are also improved.
Electrical capacitors are used for storing energy in a variety of applications. Operating voltages for such devices range from a few volts, such as those used in miniature electronic circuits, to thousands of volts, such as those used in power utility applications.
Very broadly, a capacitor has a pair of conductive "plates" or electrodes separated by a dielectric material. The conducting electrodes are typically composed of copper, silver, aluminum foil, or vacuum deposited zinc or aluminum. Capacitors utilize a variety of dielectrics ranging from ceramic materials, metal oxides, particularly tantalum oxide, plastic sheets or films, and paper.
For low and medium voltages (generally below 600 volts) metallized sheet dielectrics are commonly used. At voltages between about 200 and 600 volts, such metallized capacitors are usually partly impregnated with a dielectric liquid. The dielectric liquid penetrates the edges of a roll of sheet material from the ends of the edges of a roll of sheet material from the ends of the capacitor and any loose turns in the capacitor for filling voids and eliminating corona discharge which erodes the dielectric material.
Higher voltage capacitors are generally constructed of aluminum foil electrodes and sheet dielectric material such as a thermoplastic polypropylene and polyester film. Capacitors intended for service at more than about 600 volts are completely impregnated with a dielectric liquid with good gas absorbing properties. Typical dielectric liquids with good gas absorbing properties are PXE (phenyl xylyl ethane), MIPB (mono isopropyl biphenyl), DOP (dioctyl phthalate), castor oil, polypropylene glycol and mineral oil.
High voltage capacitors have not been able to take advantage of the self healing properties of metallized electrodes. The dielectric liquid causes swelling of the thermoplastic sheets, which causes cracking of the thin film metal electrode and an open circuit quickly develops. Dry metallized capacitors are subject to corrosion of the metallized zinc or aluminum electrodes in humid environments. Protection from such corrosion is desirable.
U.S. Pat. No. 4,842,893 discloses coating of unstated substrates with a film of cured polyfunctional acrylate and a deposit of aluminum. It is stated that the thin film coating may be useful for food packaging or as a protective coating for metal or other substrates.
The technology described in U.S. Pat. No. 4,842,893 is employed in additional patents such as U.S. Pat. Nos. 4,499,520, 4,584,628, 4,618,911, 4,682,565, 5,018,048, 5,032,461 and 5,125,138 for making monolithic capacitors. These have alternating layers of metal capacitor electrodes and deposited acrylate polymer dielectric. In a monolithic capacitor there is a layer of dielectric material deposited between layers of metal. There is no substrate between the metal layers.
One of the advantages of a thin film capacitor is its self healing ability in the event of a short circuit between electrodes. A short circuit generates a tiny arc which vaporizes metal and dielectric. In a monolithic capacitor, the vaporized material has limited chance to escape and the capacitor may be damaged.
It would be desirable to have wound capacitors made with metallized film for higher voltages where self healing can occur, and often where it is necessary that the capacitor be liquid filled. A wound capacitor has one or more sheets of dielectric coated with a metal layer wrapped into a cylinder to form a capacitor.
Many products, including many food products, are packaged in thin plastic sheet bags or the like. The thin films are desirably resistant to permeation by oxygen, water vapor and odorous gases. This can, for example, be important for protecting a food from environmental gases and also for retaining the aroma of food as it is stored.
Such barrier films are commonly made of costly plastics because less costly films are too permeable to oxygen or water to give a long shelf life. Reduced cost barrier films are highly desirable.
U.S. Pat. No. 5,021,298 describes coating of a polyolefin film substrate with a smooth layer of any plastic except polyvinylidene chloride and then vacuum metallizing the plastic so that the metal forms a barrier film. It is not necessary that the plastic itself be a barrier material. It would be desirable, however, to enhance the resistance of such a sheet to permeation by environmental gases, and also to provide protection for the metal against corrosion or the like.